In recent years, interest has grown in the use of millimeter-waves for imaging applications. Millimeter-waves are electromagnetic radiation characterized by wavelengths in the range of from 1 to 10 millimeters and having corresponding frequencies in the range of 300 GHz to 30 GHz. Millimeter-waves have the capability of passing through some types of objects which would stop or significantly attenuate the transmission of electromagnetic radiation of other wavelengths and frequencies. For example, millimeter-waves pass through clothing with only moderate attenuation, are capable of penetrating slight depths of soil, and are not obscured or adversely influenced by fog, cloud cover and some other types of visually-obscuring meteorological conditions.
Dielectric materials such as plastics, ceramics, and organic materials will cause some reflection of the waves, and some transmission, so they will be seen as partially transparent. Millimeter-waves are non-ionizing, and effective imaging systems can be operated at extremely low power levels. The IEEE standard for power density levels in this frequency range is less than 10 mW/cm2.
Generally, a millimeter-wave imaging system includes a lens or equivalent focusing element used to focus radiation from the field of view onto a two-dimensional array of imaging elements disposed in the image plane of the lens. Each array element provides a continuous electrical signal responsive to the radiation incident thereon. The output signals of the detectors illustratively are used to drive a video display unit wherein each picture element (pixel) of the displayed image represents radiation from the portion of the image incident on a given detector. That is, the image formed by the lens on the detector array is converted to signal outputs from individual detectors, which are mapped one-to-one to corresponding pixels of a video display.
Millimeter-waves travel, or propagate, through space and thus, are generally directed or guided by an antenna. The antenna contains components such as millimeter-wave stripline circuitry. A receiving antenna receives millimeter-wave radiation and directs the radiation to appropriate instruments for further processing. The transmitting antenna works in reverse fashion.
Various millimeter-wave imaging systems have been studied and developed in industry. In the research article “Phase calibration of arrays at optical and millimeter-wavelengths,” J. Opt. Soc. Am. A 13, 1593-1600 (1996), by M. Blanchard, A. H. Greenaway, R. N. Anderton, and R. Appleby, the authors designed aperture experiments with the use of optical and millimeter-wavelengths to synthesize quality images with an array of antennas. They noted that phase calibration of arrays is important to produce images of consistently good quality. Therefore, their approach to phase calibration was that of redundant spacings calibration (RSC), which can be applied at any electromagnetic wavelength. However, their experimental technique has proven difficult and expensive to implement.
In particular, millimeter-wave frequencies, in the form of electronic signals, are generally transmitted in rigid waveguide or stripline structures, which may be difficult to handle, costly, bulky, and heavy. Also, frequency conversion of modulation signals to and from a millimeter-wave carrier is generally done in several discrete stages, due to the bandwidth limitations of electronic mixers. This complicates the construction of a millimeter-wave transmitter or receiver. Further, it is generally impractical to transmit millimeter-wave signals for long distances on metallic waveguides.
Consequently, in some conventional systems, up-conversion may be performed in close proximity to the radiating aperture, and a lower-frequency intermediate frequency (IF) may often be transported on coaxial cables to and from the antenna site. As a result, a stable multiplier chain would generally be located in close proximity to the antenna aperture and supplied with a stable frequency reference. Thus, although there are many advantages of using millimeter-wave frequencies, this type of system architecture may be fundamentally incompatible with the harsh environments that antennas often endure.
Research studies have also reported on the technique of optical up-conversion in imaging systems. In up-conversion, an object under investigation may be illuminated with radiation at a first frequency, and the image beam carrying the image information may be converted to a higher frequency at which it is more amenable to detection and processing.
Another technique, in an attempt to resolve problems with imaging system, is the use of a millimeter-wave analog of an infrared (IR) focal plane array (FPA) or scanned staring systems. However, such systems require a volumetric increase in imager size and, subsequently, weight to improve imager resolution. However, the FPA approach may use very long, e.g., minutes, integration times. Further, the millimeter-wave FPA approach may not provide an economically viable solution for millimeter-wave imaging. Thus, such systems are largely impractical for many applications.
Distributed aperture approaches which synthetically reproduce image data from an array of detectors are currently under development for millimeter-wave sounding applications. Image reconstruction for distributed imaging methodologies requires the capture of both magnitude and phase of millimeter-wave field at each element of the array. Additionally, captured field information must be post-processed with large correlation engines to recover the original scene. Current distributed aperture images systems utilize distributed local oscillators (LO) and mixers to down-convert the captured field data to low intermediate frequency where it is digitally recorded. Subsequent, cross-correlators are required to regenerate the image data.
Therefore, in light of the above system requirements, it would be desirable develop a millimeter-wave imaging system with the use of optical up-conversion of the millimeter-wave signals, which does not require expensive correlators and time consuming post-processing, and enables the use of lightweight, low loss optical fibers to route signals.
Further, phase calibration or phase control (also referred to as phase locking) has been proposed in research. For instance, some systems may employ some type of phase control that aligns the phase of each of the beams in individual fibers to provide a coherent beam. Typically, the phase of each beam in each fiber may be adjusted in order to phase-lock each beam to a common reference beam. Known coherent fiber array lasers are generally continuous-wave (CW) lasers where each of the individual fiber beams is on for a period of time that is long enough to measure the phase of the fiber beams, and to adjust the phase of each beam to phase-lock to the reference beam.
In an article by Yu et al., “Coherent beam combining of large number of PM fibres in 2-D fibre array,” Electronic Letters, Aug. 31, 2006, Vol. 42, No. 18, pp 1024-1025, phase control of a fiber array using a CCD camera and a tilted reference beam was noted as having been demonstrated in research. However, the technique of Yu et al. is hampered by the refresh rate of the camera, lower phase precision and difficulty inducing different relative phases on each channel.
In addition to the limitations of the background art discussed above, new techniques to lock the phase of each of the channel's optical carrier may be helpful or desirable. In particular, phase locking of each of the channel optical carrier may be useful to preserve the detected millimeter-wave phase and allow for the recreation of the millimeter-wave image.